RFR(M,v3): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo (original) (raw)

Derek White derek.white at oracle.com
Wed May 13 21:52:33 UTC 2015

• Previous message: RFR(M,v7): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo
• Next message: RFR(M,v3): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo
• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

Hi Dmitry,

Some review comments below...

On 5/12/15 1:10 PM, Dmitry Samersoff wrote:

Everybody,

Updated version: http://cr.openjdk.java.net/~dsamersoff/JDK-8059036/webrev.03/ Now it iterates over queue and output result sorted by number of instances. FinalReference.java

• Update copyright to 2015.

ReferenceQueue.java

• Update copyright to 2015.

class "mutableInt":

• Should be "MutableInt"
• Not a collections lawyer, but should MutableInt implement Comparable?

countInstances():

• Javadoc should mention may return null.

• Can countInstances() be package-private?

• After you get the lock in line 138, you should recheck for "head == null", and return null if so. Otherwise it might sometimes return null and sometimes return an empty map.

• BIG: Is loop missing? Should "if" at line 140 be "while"?

• Merge lines 147, 148: "} else {"

• Check for consistent line spacing.

• Derek

  -Dmitry On 2015-05-07 00:51, Derek White wrote: Hi Dmitry, Staffan,

  Lots of good comments here. On the topic of what list should be printed out, I think we should focus on objects waiting to be finalized - e.g. the contents of the ReferenceQueue. It's more of a pain to walk the ReferenceQueue, but you could add a summerizeQueue(TreeMap<String, Integer>) method, or a general iterator and lambda. A histogram of objects with finalize methods might also be interesting, but you can get most of the same information from a heap histogram (ClassHistogramDCmd, and search for count of Finalizer instances). BTW, by only locking the ReferenceQueue, we should only be blocking the FinalizerThread from making progress (and I think there's a GC thread that runs after GC that sorts found References objects that need processing into their respective ReferenceQueues). But locking the "unfinalized" list blocks initializing any object with a finalize() method. The sorting suggestion is a nice touch. - Derek White, GC team On 5/5/15 10:40 AM, Peter Levart wrote: Hi Dmitry, Staffan,

  On 05/05/2015 12:38 PM, Staffan Larsen wrote: Dmitry,

  I think this should be reviewed on hotspot-gc and core-libs-dev as well considering the changes to Finalizer. I’m a little worried about the potentially very large String that is returned from printFinalizationQueue(). A possible different approach would be to write printFinalizationQueue() in C++ inside Hotspot. That would allow for outputting one line at a time. The output would still be saved in memory (since the stream is buffered), but at least the data is only saved once in memory, then. It would make the code a bit harder to write, so its a question of how scared we are of running out of memory. If the output is just a histogram of # of instances per class name, then it should not be too large, as there are not many different classes overriding finalize() method (I counted 72 overriddings of finalize() method in the whole jdk/src tree). I'm more concerned about the fact that while traversing the list, a lock is held, which might impact prompt finalization(). Is it acceptable for diagnostic output to impact performance and/or interfere with synchronization? It might be possible to scan the list without holding a lock for diagnostic purposes if Finalizer.remove() didn't set the removed Finalizer.next pointer to point back to itself: private void remove() { synchronized (lock) { if (unfinalized == this) { if (this.next != null) { unfinalized = this.next; } else { unfinalized = this.prev; } } if (this.next != null) { this.next.prev = this.prev; } if (this.prev != null) { this.prev.next = this.next; } // this.next = this; must not be set so that we can traverse the list unsynchronized this.prev = this; /* Indicates that this has been finalized */ } } For detecting whether a Finalizer is already processed, the 'prev' pointer could be used instead: private boolean hasBeenFinalized() { return (prev == this); } Also, to make sure a data race dereferencing 'unfinalized' in unsynchronized printFinalizationQueue() would get you a fully initialized Finalizer instance (in particular the next pointer), you would have to make the 'unfinalized' field volatile or insert an Unsafe.storeFence() before assigning to 'unfinalized': private void add() { synchronized (lock) { if (unfinalized != null) { this.next = unfinalized; unfinalized.prev = this; } // make sure a data race dereferencing 'unfinalized' // in printFinalizationQueue() does see the Finalizer // instance fully initialized // (in particular the 'next' pointer) U.storeFence(); unfinalized = this; } }

  By doing these modifications, I think you can remove synchronized(lock) {} from printFinalizationQueue(). I see you are traversing the ‘unfinalized’ list in Finalizer, whereas the old SA code for ‘-finalizerinfo' traverses the ‘queue’ list. I am not sure of the difference, perhaps someone from the GC team can help out? The 'queue' is a ReferenceQueue of Finalizer (FinalReference) instances pending processing by finalizer thread because their referents are eligible for finalization (they are not reachable any more). The 'unfinalized' is a doubly-linked list of all Finalizer instances for which their referents have not been finalized yet (including those that are still reachable and alive). The later serves two purposes: - it keeps Finalizer instances reachable until they are processed - it is a source of unfinalized instances for running-finalizers-on-exit if requested by System.runFinalizersOnExit(true); So it really depends on what one would like to see. Showing the queue may be interesting if one wants to see how the finalizer thread is coping with processing the finalize() invocations. Showing unfinalized list may be interesting if one wants to know how many live + finalization pending instances are there on the heap that override finalize() method. Regards, Peter For the output, it would be a nice touch to sort it on the number of references for each type. Perhaps outputting it more like a table (see the old code again) would also make it easier to digest. In diagnosticCommand.hpp, the new commands should override permission() and limit access: static const JavaPermission permission() { JavaPermission p = {"java.lang.management.ManagementPermission", "monitor", NULL}; return p; } The two tests don’t validate the output in any way. Would it be possible to add validation? Perhaps hard to make sure an object is on the finalizer queue… Hmm. Thanks, /Staffan

  On 5 maj 2015, at 12:01, Dmitry Samersoff <dmitry.samersoff at oracle.com> wrote: Everyone, Please review the fix: http://cr.openjdk.java.net/~dsamersoff/JDK-8059036/webrev.01/ heap dcmd outputs the same information as SIGBREAK finalizerinfo dcmd outputs a list of all classes in finalization queue with count -Dmitry -- Dmitry Samersoff Oracle Java development team, Saint Petersburg, Russia * I would love to change the world, but they won't give me the sources.

• Previous message: RFR(M,v7): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo
• Next message: RFR(M,v3): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo
• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

More information about the core-libs-dev mailing list